Metal oxides are useful for gas sensors. These materials show a change in conductivity when gas analytes are reduced or oxidized at their surface. Basic electrochemistry teaches that when an analyte molecule is oxidized its contact surface is reduced. This surface oxidation (or reduction) of the analyte gas which forms a redox reaction will introduce (or remove) electrons into (or from) the conduction band of the metal oxide. This reaction produces a change in the mobile charge carrier concentration within the oxide and thus a change in its electronic conductivity. The metal oxide conductivity can either increase or decrease and depends on its electronic structure and the particular analyte with which it is reacting.
This reaction usually takes place with an adsorbed oxygen species and/or defect sites within the surface structure of the metal oxide. This allows the sensor to refresh itself in ambient air as oxygen can re-adsorb after a sensing event takes place.
A problem with these metal oxide materials is their susceptibility to damage when very chemically aggressive analytes are present. Manufacturing and industrial process control, environmental monitoring, health and safety, and pollution control each have requirements for gas sensors which can withstand exposure to dangerous and chemically reactive analytes. These analytes might include acids, bases, and particular noxious chemicals. Specific examples are HCl, HF, NOx, NH3, N2H4, and KOH. These chemicals react with the sensor material surface and remove oxygen or metals from its crystal structure through formation of stable compounds with a high bond strength or kinetics faster than the refresh mechanism. This implies that the metal oxides cannot sense noxious chemicals without suffering irreversible material damage.
In the presence of chemically-reactive, noxious chemicals, metal oxide gas sensors suffer irreversible damage. This damage can manifest itself as the removal (etching) of the metal oxide from the sensor surface. These are the same chemical reactions used in CMOS processing labs to etch wafers and process levels to correct thickness.
Etch resistant layers are commonly found in CMOS processing. These materials might include nitrides such as silicon nitride (Si3N4). This insulator is commonly used as a passivation layer and dielectric in electronic materials applications. These nitrides would not work for conductimetric (measurement of a change in conductivity) sensing applications due to their electronically insulating nature.